This invention relates to processes for preparing improved toners, and developers, and more specifically to the treatment or grafting of pigments used in toners, which treatment aids in preventing the inhibition of the polymerization reaction, while at the same time maintaining the dispersibility of the pigment in the monomer used for making the toner resin.
The formation and development of images on the surface of photoconductive surfaces is well-known as documented in many prior art patents, including U.S. Pat. No. 2,297,691. The latent image formed on the photoreceptor is usually developed by applying electroscopic or toner materials to the surface of the photoreceptor. This can be followed by the transfer of the developed latent image to a suitable substrate and optionally, permanently affixing the transferred image to the substrate. Many methods are known for applying the toner particles to the latent electrostatic image including cascade development, as described in U.S. Pat. No. 2,618,552, magnetic brush development as described in U.S. Pat. No. 2,874,063, powder cloud development as described in U.S. Pat. No. 2,221,776, touchdown development as described in U.S. Pat. No. 3,166,432 and the like.
Toners have generally been prepared for example by thoroughly mixing the softened resin and pigment, thereby resulting in a uniform dispersion, by blending these ingredients in a rubber mill and then pulverizing to thereby result in the formation of small particles. The division of the resin pigment dispersions has been accomplished by jet pulverization of the material, and although this technique of toner manufacture has produced very excellent toners, it does tend to have certain disadvantages, including for example it generally produces a rather wide range of particle sizes of toner particles. Although the average particle size of toner made according to this technique generally ranges between about 5 and about 10 microns, individual particles ranging from submicron in size to above 20 microns are not infrequently produced. Additionally, this is a batch process which tends to be slow, expensive and dusty. Further, this technique of toner production imposes certain limitations upon the materials selected for the toner, as the resin pigment dispersion must be sufficiently friable in order that it may be pulverized at a economically attractable rate of production. One of the problems which arises from this requirement is that the resin pigment dispersion while being sufficiently friable for high speed pulverizing tends to form an even wider range of particle sizes during pulverization, including a relatively large percentage of dust or toner fines. Such highly friable materials are frequency subject to further pulverization or powdering, when for example they are employed as developers in xerographic copying systems. Many of the other requirements for electrophotographic developers or toners, including the requirements that they be stable in storage, non-agglomerative, possess the proper triboelectric properties for development, form good images, do not film or soil the photoconductive plate and have a low melting point for heat fusing, are only compounded by the additional requirements imposed by the above-mentioned toner forming process.
Another method of forming toner is the co-called spray drying technique wherein there is blended a water latex of the desired toner resin with a colorant, followed by spray drying the mixture to the desired particle size. The spray drying consists essentially of atomizing the colorant water latex blend into small droplets, mixing these with a gas and holding the droplets in suspension in the gas until surface tension forces cause the resin particles in each droplet to coalesce, thus encasing the colorant in that droplet. Spray dryed toners are not totally satisfactory as it is difficult to completely remove all the solvent, and the solvent which remains in the toner particles acts to affect adversely triboelectric properties between the toner and carrier, and contributes to the blocking of the toner when such is used in developing processes.
There is described in United Kingdom Patent Publication No. 1,319,815 a process for preparing toners directly from monomers by polymerization of the monomer in toner sized particles containing a colorant. A method of suspension polymerization to form toner particles is disclosed in U.S. Pat. No. 3,634,251. In the method as described in the British Patent there is prepared a kneaded oil phase component made up of one or more liquid resin monomers, coloring material, the polymerization initiator and a finely-divided inorganic dispersion stabilizer, such as a metal powder or inorganic salt or oxide, and a polar resinous additive which is soluble in a monomer. Subsequent to suspension polymerization of the monomer, if required, the finely-divided dispersion stabilizer is removed by dissolution in an acid, and the polymer particles are removed from the aqueous phase and dryed to produce toner. There are some disadvantages associated with this process, including the requirement of a high ratio of inorganic stabilizer, which needs to be removed as it will affect the properties of the toner, and thus the quality of the resulting image to be developed. A further disadvantage of such a process is that it results in incomplete polymerization of the monomer, which adversely affects the triboelectric properties, blocking properties, and fixing properties of the resulting toner.
There has been proposed a suspension polymerization process similar to the one mentioned above with respect to the British Patent but wherein the use of an inorganic stabilizer is eliminated. Generally this process is accomplished by mixing a monomer, a colorant, and an initiator to form an oil soluble organic phase, dispersing this oil soluble phase in a controlled size, of between 5 to about 20 microns in a water phase, employing a suspending agent such as polyvinyl alcohol, polymerizing, employing conventional suspension polymerization techniques, followed by introduction of a second monomer which is allowed to diffuse into the first polymer, and consequently swells the polymer; introducing a water soluble initiator, and heating this reaction mixture to effect a polymerization of the second monomer and to form the toner. The second initiator or water soluble initiator generates a free radical which attracts the surface of the swollen polymer particle and promotes polymerization at the surface by reacting with monomer at the surface thereby decreasing the monomer concentration and causing the transport of monomer to the surface by diffusion. While this process may be used to produce encapsulated toners, it does not provide an acceptable method for producing toners which are not encapsulated, and which may withstand the abrasion, stress and humidity variation to which toners are subjected to in ordinary electrophotographic development systems.
There continues to be a need for a process for preparing toners which would not involve some of the above disadvantages, including extensive processing of the polymer, colorant addition, mixing and particle formation. There is also a need for processes which would produce toner particles directly from monomer, which toner particles will have excellent triboelectric properties, abrasive resistance, blocking resistance and colorant loading capability. More importantly, there is also a need for the development of processes for preparing toners whereby the retardation-inhibition effects of pigments, such as carbon blacks, which are used in such processes, on the free radical polymerization of certain monomers can be prevented, while the dispersion of the pigment in the polymer formed during the polymerization is maintained.